1. Field of the Invention
The present invention concerns a magnetic resonance system with a radio-frequency transmission antenna and a gradient coil, of the type wherein an examination subject arranged in an examination volume can be excited to magnetic resonance at an excitation frequency by means of the radio-frequency transmission antenna, and wherein the radio-frequency transmission antenna is essentially fashioned as a resonance structure that encloses surrounds a central axis of the magnetic resonance system running within the examination volume.
2. Description of the Prior Art
Magnetic resonance systems of the above type are generally known. For example, magnetic resonance systems in which the radio-frequency transmission antenna is fashioned as a birdcage resonator fall in this category, although magnetic resonance systems with birdcage resonators are not the subject matter of the present invention.
Radio-frequency transmission antennas of magnetic resonance systems should generate a magnetic excitation field in an examination subject (usually a person), causing magnetic resonance signal to be regenerated in the examination subject. After the excitation of these magnetic resonance signals it is possible to receive them by means of suitable radio-frequency reception antennas. The radio-frequency transmission antenna can be used for reception as well.
Eddy currents that lead to an unwanted heating of the examination subject are always associated with the generation of the magnetic excitation field. These eddy currents can not be prevented. Source-like currents whose electrical fields capacitively couple in the examination subject and lead to further heating of the examination subject are also generated in addition to the unavoidable eddy currents. Such capacitive couplings in particular occur at the conductors of the radio-frequency transmission antenna, but they can also occur (albeit to a lesser extent) at local receiver coils or at cables insofar as these are located in the effective range of the radio-frequency transmission antenna.
Furthermore, in the reception of magnetic resonance signals, loss resistances are coupled both inductively and capacitively. The inductive coupling is unavoidable, but the capacitive coupling should be prevented if possible.
The problem of capacitive coupling of the examination subject to the transmission antenna occurs in a particularly severe form in radio-frequency transmission antennas that are integrated into the gradient coil. In this type of magnetic resonance system                the resonance structure has inner edges proceeding around the central axis on its inner side facing the central axis, these inner edges forming a capacitively-bridged annular gap extending parallel to the central axis, and        the radio-frequency transmission antenna is embedded into the gradient coil, such that the gradient coil at least radially outwardly and axially surrounds the radio-frequency transmission antenna, and the inner edges of the radio-frequency transmission antenna are connected in an electrically-conductive manner with inner sides of the gradient coil facing the examination volume.        
In this type of magnetic resonance system, the full transmission voltage (which can amount to several kilovolts) arises across the annular gap, which is usually part relatively narrow. Since the inner edges are connected in an electrically-conductive manner with the inner sides of the gradient coil and the inner sides are relatively large, a particularly large capacitance thus exists that leads to a large capacitive coupling. In addition, a relatively large local magnetic radio-frequency flux density also predominates immediately before the annular gap, such that particularly high local power loss densities, which can also lead to particularly significant heating, can occur by superimposition of eddy currents and capacitive currents in surface-proximate regions of the examination subject.
A direct and (without further measures) intrusive approach for minimization of the capacitive coupling is to select the distance of the radio-frequency transmission antenna from the examination subject optimally large, but this leads either to over-dimensioned radio-frequency transmission antennas or to a reduction of the spatial relationships (which are limited anyway) in the examination volume. An enlarged distance in the reception case also leads to a reduced reception sensitivity, even for local coils.
Theoretically it would be conceivable to divide the resonance capacitors (known as “multiple reduction”). In practice, however, this leads to increased capacitor losses and to an additional production expenditure. Moreover, this approach is not applicable when the radio-frequency transmission antenna is embedded into the gradient coil. The radio-frequency-sealed shielding surfaces that encase the gradient coil can not be interrupted.
It would also be possible to achieve a dissipation of the capacitive displacement currents via materials with high relative permittivity. For an appreciable effect, the thickness of such a material that must be introduced between the radio-frequency transmission antenna and the examination subject must be several centimeters, such that this solution likewise leads to over-dimensioned radio-frequency transmission antennas and a spatial constriction.